The exponential growth of the Internet has made it a ubiquitous delivery medium for a variety of applications. Such applications have in turn brought with them an increasing demand for bandwidth. As a result, service providers race to build larger and faster data centers with versatile capabilities. Meanwhile, advances in virtualization technologies have made it possible to implement a large number of virtual machines (VMs) in a data center. These virtual machines can essentially operate as physical hosts and perform a variety of functions such as Web or database servers. Because virtual machines are implemented in software, they can freely migrate to various locations. This capability allows service providers to partition and isolate physical resources (e.g., computing power and network capacity) according to customer needs, and to allocate such resources dynamically.
While virtualization brings unprecedented flexibility to service providers, the conventional layer-2 network architecture, however, tends to be rigid and cannot readily accommodate the dynamic nature of virtual machines. For example, in conventional data center architecture, hosts are often inter-connected by one or more layer-2 (e.g., Ethernet) switches to form a layer-2 broadcast domain. The physical reach of a layer-2 broadcast domain is limited by the transmission medium. As a result, different data centers are typically associated with different layer-2 broadcast domains, and multiple layer-2 broadcast domains could exist within a single data center. For a VM in one data center to communicate with a VM or a storage device in another data center, such communication would need to be carried over layer-3 networks. That is, the packets between the source and destination have to be processed and forwarded by layer-3 devices (e.g., IP routers), since the source and destination belong to different layer-2 broadcast domains. While this architecture has benefits, flat layer-2 processing has its advantages. In fact, it would be desirable to exploit the advantages of both layer-3 and layer-2 models and processing capabilities in the network.
One technique to solve the problems described above is to implement a virtual extensible local area network (VXLAN). VXLAN is a standard network virtualization technology managed by the Internet Engineering Task Force (IETF), and works by creating a logical layer-2 network that is overlaid above a layer-3 IP network. Ethernet packets generated by VMs are encapsulated in an IP header before they are transported to a remote data center where the IP header is removed and the original Ethernet packet is delivered to the destination. The IP encapsulation mechanism allows a logical layer-2 broadcast domain to be extended to an arbitrary number of remote locations, and allows different data centers or different sections of the same data center (and hence the VMs and devices therein) to be in the same layer-2 broadcast domain. The VXLAN function typically resides within a host's hypervisor, and works in conjunction with the hypervisor's virtual switch. More details of VXLAN can be found in IETF draft “VXLAN: A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks,” available at https://tools.ietf.org/html/draft-mahalingam-dutt-dcops-vxlan-02, which is incorporated by reference here.
Existing VXLAN implementations, however, cannot easily accommodate a mixture of physical hosts and VMs in the same logical layer-2 broadcast domain. This is because by default the VXLAN feature in a hypervisor encapsulates Ethernet packets generated by a VM that belongs to a certain VXLAN. A physical host, on the other hand, cannot easily participate in the same VXLAN because there is no mechanism to encapsulate the Ethernet packets it generates, which would otherwise allow the Ethernet packets to be delivered to other VXLAN-capable VMs.